14.1: Energy Metabolism

Metabolism is the set of life-sustaining chemical transformations within the cells of living organisms. The three main purposes of metabolism are the conversion of food/fuel to energy to run cellular processes, the conversion of food/fuel to building blocks for proteins, lipids, nucleic acids, and some carbohydrates, and the elimination of nitrogenous wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories: catabolism, the breaking down of organic matter, for example, by cellular respiration, and anabolism, the building up of components of cells such as proteins and nucleic acids. Usually, breaking down releases energy and building up consumes energy.

• 14.1.1: Perspectives

  We can view metabolism at several levels. At the highest level, we have nutrients, such as sugars, fatty acids and amino acids entering cells and carbon dioxide and other waste products (such as urea) exiting. Cells use the incoming materials for energy and substance to synthesize sugars, nucleotides, and other amino acids as building blocks for the carbohydrates, nucleic acids, fatty compounds, and proteins necessary for life.

• 14.1.2: Prelude to Energy Metabolism

  The insulin receptor is located in the cell membrane and consists of four polypeptide chains: two identical chains called α chains and two identical chains called β chains. The α chains, positioned on the outer surface of the membrane, consist of 735 amino acids each and contain the binding site for insulin. The β chains are integral membrane proteins, each composed of 620 amino acids.

• 14.1.3: ATP- the Universal Energy Currency

  The hydrolysis of ATP releases energy that can be used for cellular processes that require energy.

• 14.1.4: Stage I of Catabolism

  During digestion, carbohydrates are broken down into monosaccharides, proteins are broken down into amino acids, and triglycerides are broken down into glycerol and fatty acids. Most of the digestion reactions occur in the small intestine.

• 14.1.5: Stage II of Carbohydrate Catabolism

  The monosaccharide glucose is broken down through a series of enzyme-catalyzed reactions known as glycolysis. For each molecule of glucose that is broken down, two molecules of pyruvate, two molecules of ATP, and two molecules of NADH are produced. In the absence of oxygen, pyruvate is converted to lactate, and NADH is reoxidized to NAD+. In the presence of oxygen, pyruvate is converted to acetyl-CoA and then enters the citric acid cycle. More ATP can be formed from the breakdown of glucose.

• 14.1.6: Glycolysis

  Glycolysis, which literally means “breakdown of sugar," is a catabolic process in which six-carbon sugars (hexoses) are oxidized and broken down into pyruvate molecules. The corresponding anabolic pathway by which glucose is synthesized is termed gluconeogenesis. Both glycolysis and gluconeogenesis are not major oxidative/reductive processes by themselves, with one step in each one involving loss/gain of electrons, but the product of glycolysis, pyruvate, can be completely oxidized to CO₂.

• 14.1.7: Gluconeogenesis

  The anabolic counterpart to glycolysis is gluconeogenesis, which occurs mostly in the cells of the liver and kidney. In seven of the eleven reactions of gluconeogenesis (starting from pyruvate), the same enzymes are used as in glycolysis, but the reaction directions are reversed. Notably, the ΔG values of these reactions in the cell are typically near zero, meaning their direction can be readily controlled by changing substrate and product concentrations.

• 14.1.8: Pentose Phospate Pathway

  Portions of the PPP are similar to the Calvin Cycle of plants, also known as the dark reactions of photosynthesis. We discuss these reactions separately in the next section. The primary functions of the PPP are to produce NADPH (for use in anabolic reductions), ribose-5-phosphate (for making nucleotides), and erythrose-4-phosphate (for making aromatic amino acids). Three molecular intermediates of glycolysis can funnel into PPP (or be used as usual in glycolysis).

• 14.1.9: Cori Cycle and Citric Acid Cycle

  The primary catabolic pathway in the body is the citric acid cycle because it is here that oxidation to carbon dioxide occurs for breakdown products of the cell’s major building blocks - sugars, fatty acids, amino acids. The pathway is cyclic and thus, doesn’t really have a starting or ending point. All of the reactions occur in the mitochondrion, though one enzyme is embedded in the organelle’s membrane.

• 14.1.10: Stage III of Catabolism

  The acetyl group of acetyl-CoA enters the citric acid cycle. For each acetyl-CoA that enters the citric acid cycle, 2 molecules of carbon dioxide, 3 molecules of NADH, 1 molecule of ATP, and 1 molecule of FADH2 are produced. The reduced coenzymes  produced by the citric acid cycle are reoxidized by the reactions of the electron transport chain. This series of reactions also produces a pH gradient across the inner mitochondrial membrane that drives the synthesis of ATP from ADP.

• 14.1.11: Cellular Phosphorylations

  Formation of triphosphates is essential to meet the cell’s immediate energy needs for synthesis, motion, and signaling. In a given day, an average human being uses more than their body weight in triphosphates. Since triphosphates are the “currency” that meet immediate needs of the cell, it is important to understand how triphosphates are made. There are three phosphorylation mechanisms – 1) substrate level; 2) oxidative; and 3) photophosphorylation. We consider them here individually.

• 14.1.12: Energy Efficiency

• 14.1.13: Metabolic Controls of Energy

• 14.1.14: Molecular Backups for Muscles

• 14.1.E: Energy Metabolism (Exercises)

  Problems and select solutions for the chapter.

• 14.1.S: Energy Metabolism (Summary)

  To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the chapter.